If your data have been projected into a #projected coordinate system you should have no problem doing the math.
All #gpx files I've worked with include a timestamp for each time the receiver picked up the #gnss signal, typically 1/second. Since you know the location at a point in time, and you know the distance traveled, to solve for speed you divide the distance (miles or km) by the time (hours).
If you need specific help with the steps, please PM me.
